Homework Help:
Short optics question

Shiny lawn spheres placed on pedestals are convex mirrors. One such sphere has a diameter of 42 cm. A 13 cm robin sits in a tree 2.1 m from the sphere.
(a) Where is the image of the robin? in cm
b) How long is the robin's image? in cm

1/f=1/do+1/di
hi/ho=-di/do

Ok, I know that radius of a concave lens is 2f but when i use that principle i do not get the right answer...what i did is:
42/2=21=radius
21/2=10.5=focal length
1/10.5=1/210+1/x
x=11.0526 which was my answer for a which webassign says is wrong
once i get this answer part b is very easy as all i will have to do is x/13=-answer to part a/210
plz help me figure out what i did wrong im assuming that radius of a convex lens which is what is used in problem is not 2f but i have no clue what it is then and i tried googling it but could not find anything...thanks